A sheet of glass can be tempered to both increase its mechanical strength and improve its breakage characteristics. When tempered glass is broken, it collapses into a large number of relatively small, dull pieces instead of shattering into sharp pieces as in the case of annealed glass.
In a glass tempering system having a roller conveyor, the glass tempering process generally involves two steps. First, a sheet of glass is heated in a glass heating furnace to its deformation point of about 1200.degree. F. to 1300.degree. F. Secondly, the heated glass sheet is removed from the furnace and rapidly quenched by the application of cooling gas directed onto the hot glass sheet. Rapid cooling sets up high compressive forces near the surfaces of the glass sheet which give it desired strength and breakage characteristics.
The quenching operation takes place on the roller conveyor almost immediately after the glass has left the glass heating furnace. A thin glass sheet requires a more rapid application of cooling gas after exiting the furnace than does a thicker glass sheet. The close proximity of the furnace and quenching station combined with the heating step being followed directly by the quenching step can result in cooling gases which flow off the glass sheet during the quenching step entering the furnace.
Cooling gas which enters the furnace is at a significantly lower temperature than the normal operating temperature within the furnace. As a consequence, the furnace may undergo undesirable temperature swings. Also, cooling gas may prematurely cool the glass sheet in the furnace before reaching the quenching station. Premature cooling of the glass can adversely affect its tempered characteristics or shape and is especially a problem with relatively thin glass which requires more quench gases to temper properly than does thicker glass.